Tread for a pneumatic tire including aquachannel

ABSTRACT

A tread  2  for a pneumatic tire  1  has a central portion defining two circumferentially extending center ribs  22, 23  having an axial width RW, and RN 2  ranging from 5% to 25% of the tread width TW. The centermost circumferentially extending groove  8  has on both sides a tapered groove wall edges which form with a plane parallel to the equational plane acute angles α 1  and α 2  ranging between 1 and 10°. The tread height near both edges of the centermost groove extend radially above the average tread height a maximum extra tread height of between 0.3 mm and 1.7 mm. The extra tread height diminishes progressively from the groove edges P 0  P 1  toward the respective shoulder  3,4  a distance not exceeding 10% of the treadwidth.

TECHNICAL FIELD

The present invention relates to a tread for a pneumatic radial tire.

BACKGROUND ART

The tread portion of a pneumatic tire generally comprises an elastomericmaterial having a plurality of grooves therein defining ground engagingrubber elements. The particular size and shape of these elementscontributes significantly to the overall performance of the tire. Tiretreads are generally designed to provide a particular performance, suchas for instance winter, high traction or high speed performance. Thusobtaining of one particular performance characteristic is often at oddswith obtaining another one.

Hydroplaning of tires on wet pavement has long been a problem in theprior art.

Various tire designs, adapted to channel water away from the tire, andthereby maintain rubber contact with the road have been created.Although prior art rain tire designs, such as described e.g. in EP-A-508090, have improved wet traction, it is a continuing goal in the art tofurther improve such.

French patent 1 214 717 teaches a way to improve road adherence byproviding the tread pattern with raised block elements. The raisedelements are described as acting as elastic, heavily compressedprojections entering the small holes in the road surface and improvingthereby the grip of the tire. U.S. Pat. No. 5,343,918 discloses a tiretread with at least one circumferentially extending row of highstiffness blocks and one of low stiffness blocks.

The low stiffness blocks have a radial height between 4% and 10%superior to the radial height of the high stiffness blocks. The objectis to obtain uniform handling response and to improve grip.

In French Patent FR782332A teaches that the tread height near one edgeof a centermost groove extends above the average tread height to improvetraction.

In the Japanese Patent Abstract JP05338412A to Sumitomo Rubber Ind LTD,it teaches the groove wall edges can be acute angles to improve theuseful life of the tire.

In the most relevant document EP-A-0-508 090 the features recited in thepreamble of the claim are disclosed. Those features after the preamblebeing the novel features not found in the art in the combinationclaimed. The dependent claims representing features of the variouspreferred embodiments of the invention.

It is an object of the present invention to provide a pneumatic tirehaving improved wet traction while maintaining good handling, noise andirregular wear characteristics.

Other objects of the invention will be apparent from the followingdescription and claims.

Disclosure of Invention

A tread 2 for a pneumatic tire 1 has elastomeric material. The tread 2has a central portion and two shoulders 3, 4. The tread defines ribs orrows of lugs.

The central portion includes three laterally spaced circumferentiallyextending grooves 7, 8, 9, defining two circumferentially extendingcenter ribs 22, 23 having an axial width RW ranging from 5% to 25% ofthe tread width TW of the ground engaging tread portion.

The centermost circumferentially extending groove 8 has on both sidestapered groove sidewalls which form with a radial plane NP acute anglesα₁ and α₂ ranging between 1 and 10 degrees, preferably between 2 and 7degrees, more preferably between 3 and 5 degrees. The tread height nearboth edges P₀,P₂ of the centermost groove extending radially above theaverage tread height a maximum extra tread height comprised of 0.3 mmand 1.7 mm most preferably between 0.8 and 1.2 mm. This extra treadheight diminishes progressively from the groove edges towards therespective nearest shoulder of the tire over a distance not exceeding10% of the tread width.

Preferably the circumferentially extending center ribs 22, 23 are cut bysnipes 14 and may include blind grooves 52, 53.

The width of the circumferentially extending centermost groove at 8, 53,68, 77 the tread surface is between 5 and 20 mm and the groove is spacedfrom the equatorial plane a distance equaling at most 25% of thetreadwidth TW.

The tread 2 is delimited by a first and a second shoulder 3, 4. Thecentral portion includes three laterally spaced circumferentiallyextending grooves defining two circumferentially extending center ribshaving an axial width RW ranging from about 5% to 25% of the treadwidthTW of the ground engaging tread portion.

The centermost circumferentially extending groove has on both sidestapered groove sidewalls, which form with a plane parallel to theequatorial plane acute angles α₁ and α₂ ranging between 1 and 10degrees.

In an axial cross section of the tread 2, the radially outer treadsurface, on a first side and second side of the centermost groove, hasat least a first and at least a second radius of curvature R₁ and R₂respectively. The first radius of curvature R₁ has its center C₁radially outwardly of the tread surface, in a plane parallel to theequatorial plane and located between the first side of the center grooveand the fist shoulder of the tire. R₁ extends from a point P₀ on theedge the groove wall forms with the tread surface to a point P₁ locateda distance between 5% and 15% of the treadwidth, defining thereby afirst radially outer concave tread surface.

The second radius of curvature R₂, has its center C₂ radially outwardlyof the tread surface, in a plane parallel to the equatorial plane andlocated between the second side of the center groove and the secondshoulder of the tire. R₂ extends from a point P₂ on the edge the groovewall forms with the tread surface to a point P₃ located a distancebetween 5% and 15% of the treadwidth, defining thereby a second radiallyouter concave tread surface.

The centers of curvature C₁ and C₂ lie preferably in a plane locatedfrom the respective edges of the centermost groove they define, adistance ranging between 5% and 10% of the treadwidth. The point P₁ isspaced from P₀ a distance of about 7% of the tread width and the pointP₃ is spaced from P₂ a distance of about 7% of the treadwidth.

Definitions

“Aspect Ratio” of the tire means the ratio of its section height to itssection width.

“Bead” means that part of the tire comprising an annular tensile memberwrapped by ply cords and shaped, with or without other reinforcementelements such as flippers, chippers, apexes, toe guards and chaffers, tofit the design rim.

“Carcass” means the tire structure apart from the belt structure, tread,undertread, and sidewall rubber over the plies, but including the beads.

“Crown” refers to the circumferentially outermost portion of the carcasssubstantially within the width limits of the tread.

“Design rim” means a rim having a specified Configuration and width.

“Directional tread” refers to a tread design which has a preferreddirection of rotation in the forward direction of travel.

“Equatorial plane (EP)” means the plane Perpendicular to the tire's axisof rotation and passing through the center of its tread.

“Footprint” means the contact patch or area of contact of the tire treadwith a flat surface at zero speed and under design load and pressure.

“Footprint Net-to-gross” refers to the actual footprint of a deflectedtire and is the ratio of the ground contacting surface area of the treadto the total tread footprint area including the groove void area.

“Groove” means an elongated void area in a tread that may extendcircumferentially or laterally about the tread in a straight, curved, orzig-zag manner. Grooves ordinarily remain open in the tire footprint.Circumferentially and laterally extending grooves sometimes have commonportions and may be sub-classified as “wide” or “narrow”. Grooves may beof varying depths in a tire. If such narrow or wide grooves are ofsubstantially reduced depth as compared to wide circumferential grooveswhich they interconnect, they are regarded as forming “tie bars” tendingto maintain a rib-like character in the tread region involved.

“Sipes” refers to small slots molded into ribs or lugs of a tire thatsubdivides the tread surface and improves traction characteristics.Sipes tend to close completely in a tire footprint.

“Lugs” refer to discontinuous radial rows of tread rubber in directcontact with the. road surface.

“Normal load and inflation pressure” refers to the specific designinflation pressure and load assigned by the appropriate standardsorganization for the design rim and service condition for a tire ofspecific size. Examples of standards are the Tire and Rim AssociationManual and the European Tire and Rim Technical Organization.

“Pneumatic tire” means a laminated mechanical device of generallytoroidal shape (usually an open-torus) having beads and a tread and madeof rubber, chemicals, fabric and steel or other materials. When mountedon the wheel of a motor vehicle, the tire through its tread providestraction and contains the fluid that sustains the vehicle load.

“Radial” and “radially” are used to mean directions radially toward oraway from the axis of rotation of the tire.

“Rib” means a circumferentially extending strip of rubber on the treadwhich is defined by at least one circumferential “wide groove” andeither a second such groove or a lateral edge of the tread, the strip ofrubber being laterally undivided by full-depth narrow or wide grooves.“Sidewall” means that portion of a tire between the tread and the bead.

“Tread” means a molded rubber component which, when bonded to a tirecasing, includes that portion of the tire that comes into contact withthe road when the tire is normally inflated and under normal load.

“Tread width” means the arc length of the road contacting tread surfacein the axial direction, that is, in a plane parallel to the axis ofrotation of the tire.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a tire embodying a tread made inaccordance with the present invention.

FIG. 2 shows a footprint of the tread of the tire of FIG. 1.

FIG. 3 is a cross-section of the footprint shown in FIG. 2, along theline III—III.

FIG. 4 is a detailed cross-section of the center part of the tread ofFIG. 1.

FIG. 5 is a footprint of a directional tread according to the invention.

FIG. 6 shows the footprints of two asymmetrical directional tire treadsaccording to the invention.

FIG. 7 is a footprint of a high performance low aspect ratio tireaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, there is represented a pneumatic tire 1including in its crown region a tread 2 made in accordance with thepresent invention. The tire 1 is of the radial type construction anddesigned for use on passenger vehicles. The tread 2 is delimited by afirst shoulder 3 and a second shoulder 4. Sidewalls 5 connect theshoulders 3, 4 to the beads 6. The tread 2 includes threecircumferentially extending straight grooves 7, 8, 9 and two sets oflaterally extending grooves 20 and 21. The various grooves arerespectively laterally and circumferentially spaced apart across thesurface of the tread and divide the tread into circumferentiallyextending ribs 22 and 23 near the equatorial plane EP and rows of blocks11, 12 near the shoulders 3, 4 of the tire.

The two ribs 22, 23 have respectively an axial width RW₁ and RW₂ andextend substantially an equal distance on either side of the equatorialplane EP. The widths RW, range each from 5% to 25% and preferably from10% to 15% of the tread width TW of the ground engaging tread portion.The width of the circumferentially extending centermost groove 8 at thetread surface is between 5 and 20 mm.

The laterally extending grooves 20 and 21 near the shoulders 3, 4 aredisposed circumferentially about the tire in a repetitive manner, theirwidths and inclinations depending on the pitch variation, as ispracticed in the industry for noise reduction. The grooves 20 and 21 mayform opposed angles with the equatorial plane so as to give adirectional tire, as e.g. represented in FIG. 5. or—as represented—thegrooves 20 and 21 may make complementary angles with the equatorialplane so as to provide a non directional tire.

In a preferred embodiment, the ribs 22, 23 are cut by narrow grooves,i.e. grooves having a width ranging between 0.5% to 2% of thetreadwidth, or by sipes 14. The narrow grooves or sipes 14 havepreferably an inclination with respect to the equatorial plane rangingbetween 30 and 60 degrees.

Though the invention may be implemented in any passenger and light trucktire, low aspect ratio tires where the length of the contact patch inthe footprint is smaller than the treadwidth TW are of special interest.In tires having aspect ratios below 60 the invention provides a mostdesirable increase of the hydroplaning speed.

In order to facilitate the flow of water out of the middle portion ofthe contact patch of the tire footprint, the centermostcircumferentially extending groove 8, which is either cut by, or isspaced from the equatorial plane E;P a distance equaling at most 25% ofthe width, preferably at most 10%, has on both sides tapered groovesidewalls. Such can be distinguished in FIG. 3. In an axial section ofthe tire, the angles α₁, α₂ formed respectively by the uppermost portionof a first and second lateral side 15, 16 of the groove 8 with a planeNP parallel to the equatorial plane are acute, so that the distancebetween the opposing sidewalls of the groove diminishes radiallyoutwardly. The angles α₁ range between 1 and 10 degrees, and arepreferably between 2 and 7 degrees and more preferably between 3 and 5degrees. The groove walls form with a plane NP parallel to theequatorial plane, such negative taper angle from the tread surface of anew tire down to 1-3 mm above the groove bottom. In a preferredembodiment of the invention the angles formed by groove portions on thesame sidewall at different radial heights are substantially similar. Incase the center groove 5 is not located in the middle of the tread,choosing different taper angles for the opposing sidewalls might be ofadvantage.

Reference is now more specifically made to FIG. 4. Jointly with thenegatively by tapered groove wall angles α₁, the tread height near bothedges P₀,P₂ of the centermost grove 8 extends above the average treadheight TH and this extra tread height diminishes progressively from thegrove edges P₀,P₂ towards the respective shoulders 3,4 of the tire on adistance not exceedingly 15%, preferably not exceeding 5% of thetreadwidth. At both centermost groove sidewalls, the tread extendsradially between 0.3 mm and 1.7 mm, preferably between 0.6 and 1.4 mmand most preferably between 0.8 and 1.2 mm above the average tradeheight TH. Put in other words, in an axial cross section of the tread,the radially outer tread surface, on a first side 15 and a second side16 of the centermost groove 8, has respectively a first and secondradius of curvature R₁, R₂ wherein the first radius of curvature R₁,having its center C₁ radially outwardly of the tread surface, in a planelocated between the first side 15 of the center groove and the firstshoulder 3 of the tire, extends from a point P₀ on the edge, the groovewall 15 forms with the tread surface, to a point P₁ located a distancebetween 5% and 15% of the treadwidth, defining thereby a first radiallyouter concave tread surface; the second radius of curvature R₁ havingits center C₂ radially outwardly of the tread surface, in a planelocated between the second side 16 of the centermost groove and thesecond shoulder 4 of the tire, extends from a point P₂ on the edge, thegroove wall 16 forms with the tread surface to a point P₃ located adistance between 5% and 15% of the treadwidth defining thereby a secondradially outer concave tread surface.

The centers of curvature C₁ and C₂ lie preferably in a plane parallel tothe equatorial plane located from the respective edges P₀ and P₁ adistance ranging between 5% and 10% of the treadwidth.

The radii of curvature, R₁ and R₂, are substantially similar and rangebetween 5 mm and 30 mm. More preferably, the radii of curvature rangebetween 10 mm and 20 mm.

In a further embodiment of the invention, the rally outer tread surfaceof the tread near the centermost groove is defined by two or threedifferent radii on one or both sides of the groove. The farther thecenters of curvature lie away from the grove wall, the longer the radiiare.

It is understood that the remaining portions of the tread surface aredefined by the usual two or three radii of curvature, having valuescomprised between 150 mm and infinity, whereby the centers of curvaturelie radially inwardly of the tread surface they define.

The effect of the design according to the invention is explained in FIG.2 where the flow of water in a tire has been represented. The raisedgroove edges 30 contact the ground or the water on the ground a fewmillimeters before the other portions of the tread. Similarly these samegroove edges are the last portions of the tread to leave the footprintand define thereby the maximum length L of the footprint.

If the road is covered by water, the groove edges 30 impart a flowingmovemento the water which flowing movement is normal to the leadingfootprint edge. As in the present case the leading footprint edgepresents a convex curvature to the water, the latter is pushed sidewardout of the middle of the footprint. This beneficial effect of theinvention based on the shape of the leading edge of the footprint isboosted by the jet of water 31, squeezed out of the centermost groove 8as the tread enters the footprint. The water jet enhances the effect ofthe convex curvature of the center portion of the leading footprint edgeand initiates lateral water flow 32 in front—and out of—the footprintThe shape or length of this water jet depends on the car speed, thewater depth and resulting water pressure buildup, the exact design andtread depth of the tire, the pavement below the water etc. Though thelength of the water jet diminishes continuously as the car speedincreases and disappears at speeds above 90 km/h, the combined effect ofthe convex leading edge of the footprint together with the water jetwill be a significant increase of the aquaplaning resistance of a tirein deep water.

FIG. 5 represents a footprint of a directional tread according to theinvention which tread is symmetrical with respect to the equatorialplane of the ire. In this embodiment of the invention, the treadincludes three circumferentially extending straight grooves, and foursets of laterally extending grooves 50, 51, 52, 53. The various groovesare respectively laterally and circumferentially spaced apart across thesurface of the tread and divide the tread into circumferentiallyextending ribs

The two ribs, which are defined by the two circumferentially extendingshoulder grooves 58, 59 and the center groove 57, are adjacent to theequatorial plane EP. The ribs include circumferentially spaced blindgrooves 52 and 53, originating in the shoulder grooves 58, 59. The blindlaterally extending grooves 50, 51 near the shoulders of the tire aredisposed circumferentially about the tire in a repetitive manner, inribs. The ribs may include sipes 54 so as to optimize noise and wearproperties of the tire; the sipes may be straight, curved or zigzag likeand they may have the same depth as the neighboring grooves or varyingdepths. In order to improve the aquaplaning resistance of thisdirectional design, the center groove 57 has a section as represented inFIG. 4.

FIG. 6 shows on its left side a footprint of an asymmetrical directionaltire tread to be mounted on the left side of a car and on its right sidea footprint of an asymmetrical directional tire tread to be mounted onthe right side of a car. The forward travel direction of the car isrepresented by an arrow. Though only two circumferentially extendinggrooves 67, 68 have been represented, three or more may be foreseen.Groove 68, having a section as represented in FIG. 4, is delimited bytwo ribs 61, 62 which, as explained above with respect to FIG. 5,include blind grooves 63, 64 and sipes. The remaining circumferentiallyextending groove or grooves cooperate with laterally extending groove soas to define rows of blocks. Groove 68, is positioned towards the outerside of the car at a distance from the equatorial plane EP comprisedbetween 10% and 30% of the treadwidth.

FIG. 7 shows a footprint of a high performance low aspect ratio tireaccording to the invention. By low aspect ratio tire is meant here atire having an aspect ratio below 55. The tread includes threecircumferentially extending grooves 76, 77, 78 wherefrom the middlegroove 77 is preferably a so-called aqua channel as e.g. described inEP-A-508 090. The rubbery material—lugs or ribs—adjacent to aqua channel77 is contoured into the aqua channel, such that in a transversesection, the aqua channel has a curved U shape (having a rounded bottomand curved sides) favoring evacuation of large amounts of water. In eachof the shoulders of the tire is located a groove, 76 respectively 78,having a cross section as shown in FIG. 4. If the road is covered bywater, the edges of the grooves 76 and 78 impart a flow to the water,which is normal to each of the leading footprint edges. The water ispushed sideward to the shoulders, out of the footprint and towards themiddle of the tire in front of the aqua channel 77.

The invention is further illustrated by the following example: A steelbelted radial carcass tire of size 225/70 R 16 has a tread width ofabout 160 mm, two center ribs and 2 rows of blocks. The rows of blocksare molded to have axial widths RW₁ and RW₃ of about 30 mm and 35 mmrespectively.

The nominal tread depth is about 9 mm, and the widths of the groovesdepend on their axial position in the tread and the pitch variation. Thecircumferentially extending grooves have a width of about 30 mm.

Two carcass plies comprise 1100/2 dtex polyester reinforcement cords andan uncured cord density of 30 ends per inch (EPI) was used in theconstruction. The belt structure of an uncured tire comprises two singlecut plies reinforced by 2+2×0.25 mm steel cords, having a cord densityof about 22 EPI, forming uncured angles of about 23° with the equatorialplane, the angles of the cables in different plies extending in oppositedirections.

The tread has a net to gross of 58% to 73% and comprises an elastomericcompound having a modulus of 4 to 16 MPa, a Shore A hardness of 60 to75, an elongation greater than 300% and a tensile strength at breakgreater than 15 MPa.

While certain representative embodiments have been described for thepurpose of illustrating the invention, it will be apparent to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the claims.

What is claimed is:
 1. A tread for a pneumatic tire comprisingelastomeric material, said tread having a central portion and twoshoulders, said tread having a ground contacting width defining atreadwidth TW, said tread including three circumferentially extendinggrooves, wherein two of the circumferentially extending grooves, and acentermost circumferentially extending groove of the circumferentiallyextending grooves, define two circumferentially extending center ribs,each center rib having an axial width RW from 5% to 25% of thetreadwidth TW of the ground contacting portion; at least one of thethree circumferentially extending grooves having on both sides taperedgroove sidewalls, which form with a radial plane (NP) angles α₁ andα₂characterized in that α₁, and α₂ are acute angles ranging between 1and 10 degrees, such that the distance between the opposing sidewalls ofthe groove diminishes radially outwardly, and the tread height near bothgroove edges of the groove having the tapered sidewalls extendingradially above the average tread height of the tread a maximum extratread height comprised between 0.3 mm and 1.7 mm, this extra treadheight diminishing progressively from the groove edges towards therespective nearest shoulder of the tire on a distance not exceeding 15%of the treadwidth (TW) and wherein the centermost groove is acircumferentially extending aqua channel cut into two equal halves bythe equatorial plane (EP).
 2. A tread for a pneumatic tire according toclaim 1, wherein the circumferentially extending center ribs are cut bysipes.
 3. A tread for a pneumatic tire according to claim 1, wherein thecircumferentially extending center ribs includes blind grooves.
 4. Atread for a pneumatic tire according to claim 1, wherein the width ofthe at least one of the two circumferentially extending grooves at thetread surface is comprised between 5 and 20 mm.
 5. A tread for apneumatic tire according to claim 1, wherein the angles α₁ range between2 and 7 degrees.
 6. A tread for a pneumatic tire according to claim 1,wherein the extra tread height at the edges of the at least one of thetwo circumferentially extending grooves, is comprised between 0.8 and1.2 mm.
 7. A tread for a pneumatic tire according to claim 1, wherein inan axial cross section of the tread, the radially outer tread surface,on a first side and second side of the at least one of the twocircumferentially extending grooves, has at least a first and at least asecond radius of curvature R₁ and R₂ respectively; wherein the firstradius of curvature R₁, having its center C₁ radially outwardly of thetread surface, in a plane parallel to the equatorial plane (EP) andlocated between the first side of the center groove and the firstshoulder of the tire extends from a point P₀ on the edge the groove wallto a point P₁ located a distance between 5% and 15% of the tread width,defining thereby a first radially outer concave tread surface; andwherein the second radius of curvature R₂, having its center C₂ radiallyoutwardly of the tread surface, in a plane parallel to the equatorialplane and located between the second side of the center groove and thesecond shoulder of the tire, extends from a point P₂ on the edge thegroove wall to a point P₃ located a distance between 5% and 15% of thetread width, defining thereby a second radially outer concave treadsurface.
 8. A tread for a pneumatic tire according to claim 7, whereinthe centers of the curvature C₁ and C₂ are located from the respectiveedges of the centermost groove they define, a distance ranging between5% and 10% of the treadwidth.
 9. A tread for a pneumatic tire accordingto claim 7, wherein the point P₁ is spaced from P₀ a distance of about7% of the tread width and the point P₃ is spaced from P₂ a distance ofabout 7% of the tread width.
 10. A tread for a pneumatic tire accordingto claim 9, wherein there are two different radii R₁ of curvature on atleast one side of the at least one of the two circumferentiallyextending grooves, the centers of curvature of said radii lying fartheraway from the respective groove wall, the longer the radii are.
 11. Atread for a pneumatic tire according to claim 1 wherein the tire has anaspect ratio below 55%.
 12. The read for the pneumatic tire according toclaim 11 wherein each shoulder defines a circumferentially extending ribwhich is cut by blind grooves which do not extend into one of thecircumferentially extending grooves.
 13. The tread for a pneumatic tireaccording to claim 1 wherein each shoulder defines a row of lugs.